This application relates to a system of employing one or more oxy-fuel burners in a rotary furnace and methods for operating such burners in a rotary furnace, to provide enhanced heat transfer and energy efficiency.
The melting performance of a conventional rotary furnace (e.g., secondary aluminum) is usually more efficient than a stationary furnace as a result of interaction between refractory and the metal bath as the furnace rotates. In particular, heat transferred to the refractory above the metal bath can be transferred to conduction and convention as the furnace rotates and the refractory becomes submerged under the metal bath. However, the flame-furnace interaction sometimes results in striations or non-uniformities of heat and temperature in the furnace, especially in a tilted rotary furnace, and especially when unmelted scrap in the furnace impedes penetration of the flame toward the rear of the furnace.
In a conventional rotary furnace system, for example as in the double-pass furnace shown in FIGS. 7, 8, and 9A-9C, a nozzle-mixed (non-premixed or diffusion) burner 311 is mounted in the charge door (front end) 102 of a furnace 10 and fires generally into the center of the furnace headspace 106 above the charge 104. For such a burner 311, combustion (the flame 312) evolves over a finite length within the furnace 100, which defines the flame length, flame structure, and energy release profile. The mixing and combustion of fuel with oxidizer is complete when the flame is allowed to develop fully and unobstructed, as in FIG. 7. Because of the double-pass design of the furnace, the flue gas duct 110 is usually located on the door or at the front end 102 in close proximity to the burner 311. When the burner 311 is allowed to achieve a fully developed flame, the flame 312 extends to near the opposite (rear) end 103 of the furnace 100, and then the hot combustion gases 313 travel back to the flue gas duct 110, thereby giving significant time and exposure for heat transfer from the flame 312 to the charge 104 and the refractory lining the furnace walls 108.
However, when large chunks 105 of scrap, ingots, or dross are processed through a double-pass rotary melting furnace 100, they often block the evolution and development of a complete flame, as in FIG. 8. When the flame 312 is obstructed, it results in incomplete mixing and incomplete development of the flame within the furnace 100, due to short circuiting of the fuel and oxidizer (incomplete combustion products 313) to the flue gas duct 110. This, in turn, results in elevated flue gas temperatures and energy loss from the furnace 100. It also causes non-uniform heating and/or melting of the charge material 104 in the furnace 100, increasing the potential of melt losses due to overheating of the front portion 114 of the furnace 100 while leaving the back portion 118 of the furnace 100 with cold spots and buildups (resulting in loss of productivity). Typically, the firing rate of the burner 311 is erroneously increased in the attempt to “reach the back” of the furnace 100, which further exacerbates the problem.
FIGS. 9A-9C show CFD modeling results for the scenario in which a large ingot 105 or block of scrap impedes flame development. As shown in FIG. 9A, the flame 313 deflects off the front surface of the large ingot 105, causing the hottest combustion gases to remain at the front face of the ingot 105 and in the front portion 114 of the furnace 100 near the burner 311 and flue gas duct 310, while the middle portion 116 and rear portion 118 of the furnace 100 receive less flow of combustion products and less heat. FIG. 9B shows that the front surface of the ingot 105, where the flame 312 impinges, is very hot while the rear of the ingot 105 and the melt is relatively cool. FIG. 9C shows that the front 114 portion of the furnace 100 is hot, perhaps overheated, while the rear portion 118 of the furnace 118 remains relatively colder.